As is well known, in an internal combustion engine, when the opening of a throttle valve is fixed, the mass air amount drawn into the engine decreases as the air density at the upstream of the venturi decreases, and the idling rotation speed of the engine decreases accordingly. In order to maintain a preset idling rotation speed, it has been proposed to detect the idling rotation speed and control the throttle valve or a valve in a bypass path near the throttle valve such that the actual idling rotation speed is brought to the preset rotation speed. An example of this approach is disclosed in U.S. Pat. No. 3,964,457 in which the difference between the actual idle speed of the engine and a target value for the idle speed determined by the engine temperature is determined and the suction air amount to the engine is controlled such that the difference is brought to zero.
This method is effective when the idling operation continues for a certain time period, but in a transient state from an operation range of a certain speed to the idling operation, this method cannot smoothly set the idling rotation speed.
As is well known, in a high suction vacuum condition such as idling operation, the pressure upstream of the throttle valve is close to atmospheric pressure while the vacuum downstream of the throttle valve is relatively high, so that the difference between the pressures upstream and downstream of the throttle valve is very large and substantially constant and the air flow velocity through the throttle valve is approximately equal to the velocity of sound. Accordingly, the suction air flow rate is uniquely determined by the opening of the throttle valve. FIG. 1 shows a graph of engine rotation speed N versus fuel flow rate G.sub.f when the opening is fixed at a certain value in such an operation range. For example, when the throttle opening is fixed at a selected angle (e.g. .theta..sub.1) and the air flow rate (mass air flow rate) G.sub.a is fixed at a constant value G.sub.al, the engine rotation speed N increases as the fuel flow rate G.sub.f gradually increases. However, from a critical value of G.sub.f, N tends to decrease as G.sub.f increases. Usually, the air-fuel ratio (hereinafter referred to A/F) at or near the critical G.sub.f value falls within an area ranging from a stoichiometric A/F shown by a dot-and-dashed line 36 to A/F in a power zone. In a conventional engine, A/F is set at or near the stoichiometric A/F line. For example, it may be set on the A/F line shown by a broken line 38 in FIG. 1. When the operation range before deceleration is at a point n.sub.1, the throttle valve opening is changed (reduced) in the deceleration operation by an angle corresponding to the difference between the current engine rotation speed n.sub.1 and a reference engine crankshaft rotation speed n.sub.s, so that the engine rotation speed is brought to the reference engine rotation speed n.sub.s (which is a target value for the idling rotation speed). The throttle valve opening is thus controlled such that the air flow rate becomes Ga.sub.1, which causes the engine crankshaft rotation speed to be brought to the reference rotation speed n.sub.s at the current fuel flow rate. As a result, the air flow rate to the engine instantly settles to the value G.sub.al (that is, it moves to point n.sub.2) in response to the change in the throttle valve opening, because no inertia delay to the change in the throttle valve opening is included, and the engine crankshaft rotation speed is brought to the target value n.sub. s. However, the fuel flow rate to the engine does not change with a rapid response because of an inertia delay to the change in the throttle valve opening and transport delay due to the deposition of fuel on the inner wall of the suction manifold, but it gradually changes (decreases) toward the reference A/F, and the engine crankshaft rotation speed finally reaches point n.sub.3. Thus, a difference of n.sub.2 -n.sub.3 is caused in the engine rotation speed. Accordingly, the throttle valve is opened by an angle corresponding to this difference, in order to bring the difference to zero. As a result, the air flow rate instantly changes to G.sub.a5 (point n.sub.4 in FIG. 1) and the engine rotation speed changes to n.sub.s. However, the fuel flow rate G.sub.f subsequently changes (increases) to the reference air-fuel ratio delayed relative to the change in the throttle valve opening so that the engine rotation speed changes to n.sub.5, resulting in a difference of n.sub.5 -n.sub.s in the engine rotation speed. Thereafter, the engine rotation speed is controlled in a similar manner such that it changes from n.sub.5 to n.sub.6, n.sub.7, n.sub.8, n.sub.9, n.sub.10, . . . and finally converges to the reference rotation speed n.sub.s. Thus, in the above method of setting the engine rotation speed to the reference rotation speed, hunting occurs, as shown in FIG. 2, in a transient operation and a long time period is required before the engine rotation speed converges to the reference speed. If the gain of the velocity feedback loop is increased so as to shorten the convergence time, the amplitude or velocity variation range of the hunting waveform shown in FIG. 2 increases. Accordingly, the feedback loop gain must be reduced to suppress the velocity variation. This results in a long convergence time to the reference engine rotation speed. This phenomenon occurs not only in the deceleration operation but also in the starting operation in which the engine operation is shifted from a cranking condition to the idling operation.